a history of car aerodynamics

Experimental Aerodynamics

Lecture 2: A history of car aerodynamics

G. Dimitriadis

Experimental Aerodynamics Vehicle Aerodynamics


What has aerodynamics ever done for us?


It all started in Schaerbeek •! Camille Jenatzy, a Belgian race car

driver, was the first to design a car using aerodynamic principles.

La Jamais Contente by Jenatzy. It broke the 100kph barrier in 1899 and was electrically powered.

The body itself was streamlined but the driver and the wheels were not…


Alfa Romeo Ricotti

•! Built in 1914 for Count Ricotti and nicknamed ‘La bomba’.

Its geometry gave it a top speed of about 135kph.

It was a commercial failure either due to the war or due to its weird appearance.


Birth of aerodynamics •! Aerodynamics was born as a science at

the end of the 19th and beginning of the 20th century.

•! The main factors contributing to the development of aerodynamics were: –!Experimental work by Lillienthal, Langley and

the Wright brothers. –!Theoretical developments by Kutta,

Joukowski, Prandtl and others. –!Heavier than air flight by the Wright Brothers,

Santos-Dumont and others.


A bit about drag •! Aerodynamic drag is the force applied by the

wind on the car in the wind direction. •! It can be written in the form

•! where ! is the air density, V the airspeed, CD a non-dimensional drag coefficient and S the frontal area of the car.

•! The drag coefficient is assumed to be a constant in a specific range of Reynolds number values.

D =12!V 2CDS


Drag coefficient

•! The drag coefficient is important but does not determine completely the drag of a car.

•! A small drag coefficient can still give rise to high drag if the frontal area is big and vice versa.

•! The product CDS provides a more complete picture of the drag of a car.


Car streamlining

•! As aerodynamic principles became available to car engineers, the concept of streamlining was developed.

•! The shape of a falling drop of water was considered to be aerodynamically perfect.

•! Hence, several drop-shaped cars made their appearance, starting just after WWI.


Rumpler Tropfenwagen •! Edmund Rumpler was a Viennese aeronautical

engineer who designed cars after the war. •! In fact, as Germany was forbidden from building

aircraft after the war, many aero engineers converted to car design with an ‘aeronautical’ flavour.

1922 Tropfenwagen


First wind tunnel experiments •! Rumpler was also one of the first to carry

out wind tunnel experiments on cars. •! In 1922 he measured the aerodynamic

drag of a scale model of the Tropfenwagen.

•! He found that the drag of his car was about 1/3 of the drag of contemporary vehicles.

•! Nevertheless, the car was a commercial failure.


Paul Jaray

•! Paul Jaray was an Austro-Hungarian engineer who designed Zeppelin airships.

•! After the war he created many streamlined car designs.

•! His cars also featured smooth body surfaces, integrated fenders and headlamps, cambered windshield and other innovations.

•! His designs were adopted or copied by several car manufacturers, such as Audi, BMW, VW Daimler-Benz and others.


Jaray cars

1923, Audi Type K

1933, Tatra V570

1933 Mercedes Benz 200

1933 Tatra 77


Minimum drag coefficient •! Jaray and Kemplerer carried out a

series of wind tunnel tests on half-bodies in 1922.

•! The starting point was a body of revolution with a length-to-diameter ratio of 5.

•! Half-bodies based on this shape were tested close to the ground.

•! The minimum drag coefficient for a half-body with wheels was found to be equal to 0.15.

•! Subsequently, many vehicle aerodynamicists tried to create vehicles with CD=0.15.


Other streamlined cars 1923 Benz RH Tropfenwagen

1932 Auto Union Type C 1923 Bugatti Type 32

1940 Alfa Romeo 512


Comments on streamlined cars

•! Streamlined cars are based on airfoils or half airfoils.

•! These shapes were shown to have very low drag in the wind tunnel.

•! However, real cars based on these shapes had much high drag coefficients of around 0.4

•! Real cars have windows, bumpers, side mirrors, headlights, engine intakes, wheel wells, wheel fairings etc.

•! All of these elements increase the drag coefficient significantly.


Success of streamlined cars •! Streamlined cars never became really

popular. •! Airfoil shapes are not very practical or

comfortable. •! Streamlined cars remained

experimental or limited to speed record attempts.

•! There some notable exceptions.


Porsche 911

The Porsche 911 is one of the very few streamlined cars that are still in production today.


VW Beetle The Beetle was a Porsche design based on Jaray’s cars, particularly the Tatra V570. It was a pseudo-Jaray shape, because it had a much too steep rear slope. Such cars are known as ‘fastbacks’. Nearly 22 million beetles were built from 1938 to 2003.


Kammback cars •! Another German aerodynamicist,

Wunibald Kamm, imported an important aerodynamic principle into car design.

•! Airfoils with a truncated trailing edge have only slightly higher drag than complete airfoils.

•! By consequence, streamlined cars with truncated rear ends have slightly increased drag but are lighter and cheaper to build.


Kammback cars 1938 BMW Kamm-Coupé

1974 Citroën CX

1976 Porsche 924


Postwar era

•! The immediate postwar era saw a move away from aerodynamics.

•! Car design in the United States concentrated on large, spacious and comfortable cars with a ‘bathtub body’.

•! In Europe cars remained smaller due to the small European city roads and lack of parking space. Nevertheless, aerodynamics was not fashionable.


US cars of the 50s

•! Large and bathtuby •! Nevertheless, the

streamlined bumpers, headlights and wheels do decrease drag to a certain extent.


European cars of the 50s

1959 Fiat 500

1950 Citroen 2CV 1952 Mercedes Type 300

Generally small and boxy.


The petrol crisis

•! Aerodynamics came back into car design in the 1970s after the end of the postwar boom.

•! The petrol crisis in particular turned fuel efficiency into an important issue again.

•! Aerodynamic design started to improve again and the drag coefficient of the average car started decreasing.


European car CD values

Ford Probe V


US drag history Chrysler’s car line

Ford’s US car line sales-weighted average drag coefficient.

a)! Cadillac b)! Buick

GM cars


Detail optimization

•! The drag improvements of the 70s where based on detail optimization.

•! It’s based on carrying out numerous minor local modifications on order to obtain significant total drag reductions.

•! Such details are the curvature of edges and pillars, camber of panels, tapering, size and location of spoilers, side mirror fairings etc.


The devil is in the details

1974 VW Scirocco. Designed by detail optimization. Its drag coefficient was 0.41

1969 Opel GT. Designed to be streamlined. Its drag coefficient was also 0.42


Shape optimization •! Detail optimization yielded impressive results but quickly

reached its limits. •! From the late 70s onwards, there was a re-evaluation of

the work of 1930s aerodynamicists in order to obtain further drag reductions.

•! Shape optimization was introduced. It starts from a basic, low drag shape, which is gradually modified in small steps to yield a realistic car shape.

•! The first car to be designed using this approach was the Audi 100. The 1983 C3 had a drag coefficient of 0.3.

•! This car also pioneered flush windows, which helped achieve this low drag value. Such windows became standard on all cars later on.


80s shape optimization

1983 Audi 100 C3 1984 Renault Espace

1983 Ford probe IV. CD=0.15


Current state of the art •! The current state of the art in aerodynamic

design combines both shape and detail optimization.

•! A reasonable drag coefficient is around 0.25-0.3 for a modern car. Cd values of under 0.25 is a reasonable future target.

•! Some common characteristics: –! Small radiator air intakes placed near the front

stagnation point, under the bumper. –! Higher inclination angles for both front and rear

windscreens


More about drag •! Car drag can be split into two principal

components: •! External flow drag:

–! Car body –! Protuberances: mirrors etc –! Wheels and wheel wells

•! Internal flow drag: –! Engine cooling –! Heating and ventilation –! Components: brakes etc


Physics of external drag •! For external car flow, there are three

principle mechanisms that cause drag: –!Pressure drag: drag caused by differences

in pressure between the front and the back of the car.

–!Skin friction drag: drag due to the tendency of the airflow to stick to the body’s surface.

–! Interference drag: drag due to flow interference between different components of the car.


Pressure drag Pressure drag is caused by the difference between attached flow at the front of the car and detached flow at the back. The absence of a rear stagnation point means that there is a significant difference in pressure between front and back. Therefore, there is a net force opposing the motion.


More on pressure drag This smoke flow visualization photo shows clearly the differences in the nature of the flow between the front and back.

The separated flow in the wake is air that is set in motion by the car. If the car is adding momentum to the air, then it must lose momentum itself. This loss of momentum is a drag force.


Pressure distribution

Example of pressure distribution around car. The drag force is given by the integral of the pressure distribution:

d = pdS!


Vortices behind car

Typical vortical structures behind a car. The size, strength and shape of the vortices depends on the body geometry.


Car wakes are 3D structures

3D car wake from superposition of two 2D sections. Section A1 shows separation in the longitudinal direction. Section A2 shows shed vortices. The result is a U-shaped vortex.


Skin friction drag

•! This type of drag is caused by the friction between the air and the body’s surface.

•! Skin friction is generated inside the boundary layer and depends on its properties.

!w = µ"u"y y=0

Skin friction depends on the wall shear

d = !w x( )0

l" dx


More on skin friction •! Skin friction depends on whether the

boundary layer is laminar or turbulent. •! Turbulent flow causes higher values of

wall shear. A smooth body surface ensures that the boundary layer remains laminar as much as possible.

•! Skin friction, as defined here, is only possible when there is a boundary layer. When the flow is separated the definition is not valid.


Interference drag •! Interference drag is very difficult to quantify. •! As an example consider this: the drag of a car

with side mirrors is higher than the drag of the car without the mirrors plus the drag of the mirrors themselves.

•! The additional drag is caused by aerodynamic interference between the car body and the mirrors.

•! Other components causing interference: wheels, wheel wells, bumpers etc

a history of car aerodynamics

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